1. Field of the Invention
The present application relates to a polymeric foam tubing which can be heat-recovered, e.g. heat-shrunk. More specifically, it relates to an extruded polymeric foam tubing including a dispersed, encapsulated blowing agent.
2. Introduction to the Invention
Insulation, padding or a protective covering is often required for a variety of substrates, such as metallic wires, rods, pipes, or other elongated irregular shapes, such as handles and grips, in a broad range of applications, from tools and appliances to clothing. A foamed polymer coating is an effective and inexpensive method of providing such insulation or padding. In particular, a tubular foam structure can be extruded, and made into a heat shrinkable tubing. When the substrate is inserted into the foam tube and then the tube is heated above the melting point of the polymer used in the polymer composition of the tube, the heat shrinkable polymer foam tubing will recover snugly over the substrate.
Extrusion is a continuous molding process often used with polymer materials. In extrusion, a polymer material is continuously heated and mechanically worked into a homogeneous charge of viscous fluid in the melt phase. Alternatively, the polymer may be introduced into the extrusion device already in a melt phase. The heated charge is then forced through an open-ended die. The polymer is extruded from the die in the general shape of the die; often, it is an annular shape. The extruded melt is then solidified by air cooling or water quenching the extruded material. To make a heat-recoverable article, the solidified material usually is crosslinked, and then mechanically expanded. Applying heat to the expanded article causes it to recover, usually by shrinking, to approximately its original, unexpanded shape.
When a foamed extrudate is desired, the polymer melt is often mixed with a blowing agent. Chemical blowing agents typically are in the form of heat-activated fine powders, such as sodium bicarbonate or azodicarbonamide. When the polymer material containing the blowing agent powder is heated and melted, the bulk of the powder decomposes and forms pockets or balloons of gas within the structure of the polymer melt. The gas immediately dissolves into the polymer melt, and mainly stays dissolved within the melt due to the prevailing high pressure in the extruder at extrusion process conditions. However, in the conventional tube extrusion process of forming a tube of foamed polymer material, some of the dissolved gas is released into the inner cavity of the extruded polymer foam tube as the molten polymeric material exits the die. This can lead to deformities in the shape and dimensions of the final product.
Alternatively, physical blowing agents have been introduced into the polymer melt late in the extrusion process, i.e. after heating the polymer, but before the final pumping of the polymer melt through the extrusion die. Such a process still suffers from the same disadvantages as the previously described conventional process, and variations in polymer foam tube shape and dimensions are still present at unacceptable levels. Further, the foam cell size in the finished product has poor uniformity, which may adversely impact the ability of the tubing to expand in diameter following a crosslinking step.
Accordingly, there is an existing need for a composition for a polymer melt for extrusion to be used in tubing that will create a foamed polymeric material capable of being made into a useful heat shrinkable foamed structure, preferably in a continuous process, such as tube extrusion or parison blow-molding process, but not allow the gases to escape and deform the tubular structure.